The present invention relates to a process in which an admixture of toluene and C.sub.9 alkylbenzene is transalkylated to form benzene and C.sub.8 alkylbenzene. It specifically relates to fractionation of the reaction zone effluent of a process in which an admixture of toluene and C.sub.9 alkylbenzene is transalkylated to benzene and C.sub.8 alkylbenzene.
At the present time, about 90 percent of the benzene produced in the U.S. is derived from petroleum sources and the balance is derived from natural gas and coal. This represents a sharp change from as recently as 1957 when the steel and coal industries produced more benzene than did the petrochemical industry. This is in large part due to the high growth rate of benzene demand experienced during the past decade, averaging 12 percent per year during that time, but also reflects a stagnation of benzene production by tar distillers and coke oven operators. About 85 percent of benzene consumption in the U.S. now goes to production of ethylbenzene, phenol, and cyclohexane, while a relatively small amount goes to aniline, maleic anhydride, chlorobenzene and other uses. Ethylbenzene, which consumes 48 percent of U.S. benzene production, is an intermediate chemical in the production of styrene, and is prepared principally from benzene (90 percent) by alkylation with ethylene, with fractionation of mixed aromatics derived from petroleum supplying the remainder. Styrene is used principally for the production of styrene-butadiene rubber and styrene resins such as polystyrene, both the straight and rubber-modified polystyrenes finding application in consumer products such as packaging, toys, luggage, housewares, etc. Twenty percent of benzene production is used to prepare phenol, which is produced by various methods, principally by way of cumene as an intermediate, and is itself an intermediate chemical in the production of phenolic resins, which are used in molding applications, plywood bonding, laminating resins, friction materials, thermal insulation, etc. And 17 percent of benzene production goes to produce cyclohexane, an intermediate in the manufacture of nylon. Because of benzene's use in the production of consumer goods, it is expected that demand will continue strong, but perhaps not at the annual growth rate of 12 percent experienced during the past decade due to benzene's present unit cost more or less quadruple that of the previous decade.
Like benzene, demand for xylene has been strong principally due to increasing demand for paraxylene. Over the past decade, while yearly production of mixed xylene has increased 9 percent, that of orthoxylene has increased 13 percent and that of paraxylene has increased 24 percent. Xylenes are produced almost solely from petroleum, with less than 2 percent production from coaltar and coke oven light tars. Xylene isomers together with ethylbenzene as produced from petroleum are normally found as follows as related to the total C.sub.8 aromatic: ethylbenzene 15-25 percent, orthoxylene 15-25 percent, metaxylene 35-45 percent, and paraxylene 12-22 percent. As stated hereinabove, ethylbenzene is used in the production of styrene; orthoxylene is a feedstock in the production of phthalic anhydride while paraxylene is used for polyester manufacture.
Benzene, ethylbenzene, and the xylene isomers are principally prepared and separated together with toluene from petroleum by a series of processing units as follows: (1) In a crude unit, crude petroleum is fractionated into several boiling range cuts, one of which is a naphtha cut which boils in a range of about 100.degree. to 350.degree. F. (2) After depentanizing or deisohexanizing of the naphtha, it is passed to a desulfurization and reforming unit, where sulfur is removed to less than one part per million and the aromatic precursors in the naphtha are upgraded to their respective aromatics. (3) Reforming unit effluent, normally containing about 30 to 60 percent aromatics, is passed into an aromatics extraction unit, wherein normally either a glycol or sulfolane is utilized as a solvent to extract aromatic components from non-aromatic ones. Extract containing about 99.9 percent aromatics is clay treated to reduce olefin content and is separated in a fractionation zone to prepare the various aromatic components in purified streams as desired. Several important variables affect the quantity of individual aromatic products obtained by the hereinabove processing scheme, the most important of them being the quality of the crude and the market for products. Crudes vary significantly in quality in regard to aromatic and aromatic precursor content, and in regard to the ratio of aromatics and aromatic precursors by carbon number. While there is significant variation, benzene, toluene, and xylene (including ethylbenzene) are typically produced in the following ratio:
______________________________________ benzene = 1 toluene = 2.5 to 3.0 xylene and ethylbenzene = 2.0 to 2.5 ______________________________________
Although it appears that the production rates of toluene and xylene may be greater than that of benzene, toluene, and xylene U.S. production in 1973 were 1453, 936, and 818 million gallons, respectively. Consumption is far greater of benzene than toluene, or xylene, and accordingly, production of toluene and xylene is restricted, normally by one of two means. Firstly, a naphtha cut with an end point of about 230.degree.-300.degree. F. may be processed, thereby substantially reducing the toluene and xylene precursors in the reforming unit feedstock; and secondly, a dealkylation unit may be utilized to convert toluene to benzene. A further explanation of higher benzene production than toluene or xylene is related to the aromatics production from coke-oven light oils, coaltar, and pyrolysis processes, which is similar from these sources in distribution of benzene, toluene, and xylene produced. While there may be substantial variation from one producer to another, benzene, toluene and xylene comprise about 80, 15 and 5 percent, respectively, of the BTX aromatics produced in these processes.
Toluene, unlike benzene, orthoxylene, and paraxylene, does not have strong demand as an intermediate chemical in the manufacture of consumer products. End uses such as polyurethane production, aviation gasoline, or solvents require only about 35 percent of U.S. production; the remainder of U.S. toluene production is dealkylated to benzene. In the present description, U.S. production of aromatics stated hereinabove is the production and separation into relatively purified streams of said aromatics, but in fact, total production is substantially greater. For example, only about 20 percent of total toluene produced is actually separated into a purified stream, the remainder being used as a high octane gasoline blending component. As a blending component, toluene has a premium value with a research octane number of 105.8. With the present emphasis on lead-free gasoline, high octane blending components such as toluene are becoming relatively more valuable than previously, but toluene dealkylation has increased and now accounts for about 32 percent of benzene production as compared to about 22 percent in 1965. Accordingly, it is observed that toluene dealkylation is becoming an increasingly attractive route to benzene production.
Both thermal and catalytic toluene dealkylation processes are available to produce benzene. Both catalytic and thermal dealkylation are practiced in the presence of hydrogen at high reaction temperatures of about 1200.degree.-1300.degree. F. to achieve over 95 percent dealkylation to benzene. Both processes may accept feedstock including alkylaromatics higher than toluene, and these are also normally converted to benzene although mild conversion of C.sub.9 and heavier aromatics to xylene is known in the art. Feedstocks including paraffins, naphthenes, and aromatics are also provided to both dealkylation processes to result in benzene and light paraffin products, i.e., methane and ethane. Because of high hydrogen consumption and low benzene volume yields, dealkylation to benzene of alkylaromatics heavier than toluene is not as advantageous as toluene dealkylation.
A relatively new process development is a catalytic transalkylation process in which toluene is transalkylated to benzene and xylene in the presence of hydrogen. The process is advantageous as compared with dealkylation processes in the respect that hydrogen consumption is substantially reduced and reaction temperatures are less severe. In a transalkylation process in which toluene is a feedstock, the principal reaction taking place is as follows: EQU 2 C.sub.7 H.sub.8 .fwdarw. C.sub.6 H.sub.6 + C.sub.8 H.sub.10
molar yields of benzene and xylene are essentially achieved in the process, and while the theoretical volume yield of benzene from toluene is about 84 percent by dealkylation, the combined theoretical volume yields of benzene and xylene is about 100 percent from transalkylation of toluene. Of the C.sub.8 aromatic product, only 1 to 2 percent is ethylbenzene, with the xylene isomers comprising the remainder in the following proportions: para 23-25 percent, meta 50-55 percent and ortho 23-25 percent.
In addition to benzene and C.sub.8 aromatic products, about 2 to 4 percent of the reactant is converted to light hydrocarbons, such as methane and ethane, and heavy aromatics containing 10 or more carbon atoms. An alkylaromatic product containing 9 carbon atoms is produced in the reaction, but following separation of the reaction zone effluent, it may be recycled to extinction in the reaction zone. Although the primary reactant introduced into the process is toluene, C.sub.9 aromatic may also be used and upgraded principally to xylene and a C.sub.10 aromatic. For example, trimethylbenzene is principally converted to xylene and tetramethylbenzene.
The process of the present invention is applicable to a transalkylation process in which toluene reactant alone or C.sub.9 alkylbenzene alone is contemplated, but is particularly directed toward a process in which an admixture of toluene and C.sub.9 alkylbenzene is introduced into the process and is transalkylated in a reaction zone wherein complete conversion of the reactants is not achieved, thus requiring separation and recycling to the reaction zone of unreacted reactants. Application of the present invention is unique to a process in which toluene and C.sub.9 alkylbenzene in admixture are transalkylated to form benzene and C.sub.8 alkylbenzene products.